The present invention relates to an improved method for controlling temperature response of a part in a conveyorized thermal processor by means of closed-loop feedback.
Thermal processing involves a series of procedures by which an item may be exposed to a temperature-controlled environment, and is used in a variety of manufacturing procedures such as heat treating, quenching and refrigerated storage. One example of a thermal processor is a reflow oven. The production of various goods such as electronic circuit boards in solder reflow ovens frequently entails carefully controlled exposure to heating and/or cooling for specific periods. The elevated temperature conditions needed to solder component leads onto printed circuit boards must be gradually and uniformly applied to minimize thermal expansion stresses. For this reason, convection heat transfer may be employed in these solder xe2x80x9creflowxe2x80x9d operations. The connecting solder paste incorporates an amalgam of substances that must undergo phase changes at separate temperature levels. Solder reflow may be performed by sequentially passing a part (such as a printed circuit board to become a processed product) through a series of thermally isolated adjacent regions or xe2x80x9czonesxe2x80x9d in the reflow oven, the temperature of each being independently controlled.
Convection heat transfer chambers or zones are typically set to a fixed control temperature throughout a thermal process. A zone may have one or more controlled thermal elements, these each having a corresponding control monitoring location. A thermal element may be defined as either a heat source for heating or a heat sink for cooling and may be commanded to a control temperature. The temperatures commanded at the control monitoring locations form a xe2x80x9ccontrol temperature profilexe2x80x9d along the reflow oven. The temperature exposure of the part may be governed by the processor temperature of the air in each zone and the exposure time within each region. The temperature of the air along the zones forms a xe2x80x9cprocessor temperature profilexe2x80x9d. The series of instantaneous part temperature values as the part travels along the conveyor and through the oven may be called a xe2x80x9cpart temperature profilexe2x80x9d and if based on measured data may be called a xe2x80x9cmeasured part temperature profilexe2x80x9d. The temperature response of the part must satisfy a manufacturer""s part specification requirements, which include allowable tolerance bands or tolerance limits around target values that have been defined. A measured value within the corresponding tolerance limit satisfies the specification. The procedure for operating the oven to obtain temperature data (used in creating a measured part temperature profile) may be called a xe2x80x9ctest processxe2x80x9d.
The temperature response of the part may be monitored by instrumenting the part or adjacent device with one or more thermocouples (or other temperature measuring contact devices such as thermisters or resistance temperature detectors) prior to sending the part into the reflow oven or by remote observation with a thermal sensor. Alternatively, the temperature response of the part may be measured by a remote means such as an infrared or optical scanner. The thermocouple measurements can be sent to a data acquisition device through an attached cable or by a radio transmitter or by similar means. The temperature along the conveyor may also be measured by different means. Two examples are a thermocouple attached to the conveyor (though not in thermal contact) so that it moves along with the part, and a fixed probe extending along the length of the oven and positioned adjacent to the conveyor having a plurality of thermocouples disposed along the probe interior.
A thermal processor, such as a reflow oven, may be modulated by a series of n control parameters labeled Cj numbering from j=1 to n. These control parameters may include the oven setpoint temperature at each zone, the conveyor speed, product conveyor density (number of parts per unit conveyor length), or a combination of these and other variables subject to direct adjustment or indirect influence during the thermal processor operation. Other physical influences on the thermal process include initial conditions, which may depend on the ambient temperature and humidity, as well as characteristics difficult to measure directly, such as convection rate. These may be referred to in the aggregate as noncontrollable processor parameters.
A side view diagram of a reflow oven is shown in FIG. 1 as an example of a thermal processor. The reflow oven 10 has a conveyor 12 aligned along the length of It the oven 10 that moves in the direction towards the right from entrance 14 to exit 16. The oven interior may be divided into two or more zones for thermal processing. In the illustration, first and second zones 18a and 18b are shown. Each zone has at least one heating and/or cooling element 20a and 20b and may feature one or more monitoring instruments 22a and 22b in proximity to the elements to monitor thermal processing. These element monitoring instruments 22a and 22b may be thermocouples or thermostats. The oven 10 may also include one or more recirculation fans 24 to increase convection. The conveyor 12 may be moved by means of a conveyor motor 26a; the fan may be rotated by means of a fan motor 26b. The settings for heating and/or cooling elements 20a and 20b, and the motors 26a and 26b are controlled by a control station 28, which receives settings input from a receiver 30 instructed by an operator 32 or other means. Each control parameter may be commanded to a target condition at a control interface between the input receiver 30 and the control parameter. The example shown in FIG. 1 features a first zone interface 34a, a second zone interface 34b, a conveyor interface 34c and a fan control 34d. These control parameters may be expressed as C values in a series of n dimensions where each control parameter may be identified as one of C1, C2, . . . , Cn or as Cj where j=1 to n. In the example shown in FIG. 1, n equals 4. Measured data from instruments monitoring control performance such as element monitoring instruments 22a and 22b may be received by a data acquisition device 36a and recorded on a storage medium 36b. 
The response of the thermal processor to the settings of the control parameters may not be identical to the Cj values. This processor response may be characterized as processor parameters labeled Cxe2x80x2 in a similar series of n dimensions. In conjunction with the processor parameters Cxe2x80x2 may be measured and these data recorded. The correlation between the control parameters C and the process parameters Cxe2x80x2 may be influenced by the rate of convection, the thermal isolation across zones and the ability of the thermal elements to attain the target control parameter values. In practice, the process parameters may fluctuate with time around a local or running average {overscore (C)}xe2x80x2. Process deviance tolerances may be specified or defined, which if exceeded may cause the thermal processor to discontinue. These tolerances may be expressed as absolute differences between the control parameters and their corresponding average processor parameters |Cxe2x88x92{overscore (C)}xe2x80x2|j for series j=1 to n. After the thermal processor has had sufficient time (txe2x86x92∞) for processor parameters to reach thermal equilibrium (assuming the control parameters are held to fixed values), such that the process parameters Cxe2x80x2 vary only within a specified equilibrium range or tolerance xcex5Cj, that difference |Cxe2x88x92Cxe2x80x2|j may be expected to vary within that equilibrium range expressed as |Cxe2x88x92Cxe2x80x2|jxe2x89xa6xcex5Cj.
A part 38, such as a printed circuit board, may be placed on the conveyor 12 upstream of the oven entry 14 to be transported through the oven 10 and egressing through the exit 16. The time-varying thermal exposure may be obtained by using a series of adjacent first and second zones 18a and 18b at a conveyor speed such that part location in the oven 10 may be defined by conveyor speed multiplied by time since entry. Temperature of the part 38 may be monitored remotely by an infrared or optical scanner or else measured conductively by one or more attached thermal sensors such as a thermocouple 40. The measured part temperature data from the thermocouple 40 may be transmitted to the data acquisition device 36a, either by direct connection or broadcast signal, and recorded on storage medium 36b. 
The part profile specification 42 may be provided to the operator 32 to compare the part temperature profile with the specification ranges and with which to manually adjust the control parameters in the control station 28. The specification ranges represent the allowable limits for calculated feedback parameters that are selected to characterize the thermal process for the part. These feedback parameters may be written as B values in a series of m dimension where each feedback parameter may be identified as one of B1, B2, . . . Bm, or as Bi where i=1 to m. The amount by which a measured value deviates from the middle of its specification range corresponds to a feedback index, and the maximum of these feedback indices denotes the Process Window Index S for that thermal process.
Temperature response of a part to a convective heat transfer environment depends on essentially four factors: the part""s initial temperature (denoted by superscript 0), the ambient temperature of the environment to which the part may be exposed, the convection coefficient which depends on characteristics of the fluid medium and its motion, and the physical and material properties of the part itself. In a reflow oven, the ambient temperature varies with distance along the conveyor path, which to the part travelling along that path effectively varies with time t. Convective conditions may be expressed by the nondimensional quantity Nusselt number Nu. The part""s transient response may be expressed by two additional nondimensional quantities: Biot number Bi and Fourier number Fo, as is well known in the art. These three nondimensional quantities may be collectively treated as functions of fluid velocity u (which in turn may be dependent on the fan speed xcfx89), thermal conductivity k of the fluid heat transfer medium (air, in the case of a reflow oven) and of the part (which may be treated as a constant bulk property for simplicity in many applications), density xcfx81 of the fluid (subscript a) and the part (subscript p), and heat capacity c of the fluid and the part. Thus the part""s transient heat transfer response may be expressed as a function in equation (1).
Tp(t)=ƒ{Tp0, Tz(t), Nu, Bi, Fo}=ƒ{Tp0, Tz(t, uc), u(xcfx89), kp, xcfx81p, cp, ka, xcfx81a, ca, xp,}xe2x80x83xe2x80x83(1)
in which t is time, time-varying Tp is the part temperature, Tz is the processor (or thermal source) temperature, u is air velocity, uc is conveyor speed, and xp constitutes one or more characteristic length dimensions of the part which relate to wetted area and/or it volume. Another factor related to conveyor speed might be loading density of parts on the conveyor. In some circumstances, a part may have sufficient heat capacity so that several of them close together on a conveyor may absorb thermal energy so as to reduce the part temperature rise rate from that of an isolated part on the conveyor. This condition may be controlled by a parts loader to the conveyor.
Temperature variation may be compared along the reflow oven""s length between the commanded temperatures for the elements, the measured zone temperatures, the measured temperature response for the part and the corresponding specification range for the part. A graph showing temperature along the conveyor path is shown in FIG. 2. The temperature scale forming the ordinate 50 may be plotted against the abscissa 52 or distance axis along the conveyor path. The first zone 54a and second zone 54b encompasses a spatial region across a portion of the oven distance 52. The control temperature levels for each zone region can be appended to form a control profile 56 that may be piece-wise continuous along the distance axis 52. Processor temperatures measured (or otherwise determined) near the thermal elements form a processor profile 58. The processor temperature in processor profile 58 often deviates from the control temperature in control profile 56 by a small quantity under convective heat transfer due to cross flow between zones and other physical effects.
The part exhibits a temperature response based on temperature measurements over time along the oven length. This response may be plotted as a part profile 60 along the distance axis 52 by multiplying the conveyor speed by the time from the part""s entry into the oven. Air (the heat transfer medium in the oven) has a relatively low thermal conductivity (compared to the part) with which to convect heat from thermal elements to the part surface. Due to the low convection and the part""s internal heat capacity by virtue of its mass, the part response temperature along part profile 60 will lag the control temperature 56 set in the zone region to which the part may be exposed.
The part temperature profile 60 may be evaluated for particular characteristics earlier referred to as feedback parameters related to the thermal process specification. For example, the maximum temperature rise rate 62 may be determined from a discrete temperature increase 62a over a selected time interval 62b, preferably when the temperature increases relatively steadily for the selected time interval 62b. Additionally, the peak part temperature 64 may be determined by the highest measured temperature along the part temperature profile 60. Similarly, xe2x80x9ctime above reflowxe2x80x9d may be ascertained as a reflow time interval 66 by establishing the time period when the part temperature profile 60 exceeds the reflow temperature level 66a beginning at the initial time 66b and ending at final time 66c. These feedback parameter values may be compared to specified ranges that the profile must be within in order to have been properly thermally processed. The maximum temperature rise rate 62 may be compared to the rise rate range, featuring a minimum acceptable rate 68a and a maximum acceptable range 68b. Typically, a temperature rise rate may apply over a narrow and specific time interval, and associated with a particular zone. The peak part temperature 64 may be compared to the peak range bounded by a minimum accepted peak temperature 70a and a maximum accepted temperature 70b. Similarly, the reflow time interval 66 may be compared to the reflow time range, with a minimum accepted period 72a and a maximum accepted period 72b. The closer the feedback parameters conform to the middle of the ranges, the smaller its feedback index, the maximum of which yields the Process Window Index described in more detail below.
Typically in the past, the test procedure for a reflow thermal process involves having an operator set the oven controls, allow sufficient time for the processor temperatures in each zone to reach thermal equilibrium, set the conveyor speed, and send an instrumented part through the oven. Thermal equilibrium may be defined as a steady-state condition in which the processor temperature has stabilized (preferably within a defined processor deviance range around its corresponding control parameter value) and whose average does not change with time. In practice, a small fluctuation within a specified processor equilibrium range may be allowed. After a comparison between the part""s target temperatures and its measured values along the oven length, the operator guesses or estimates changes to the control parameters or the conveyor speed. The operator implements the guess and repeats the process over and over until the temperature difference between target and measurement may be reduced to an allowable tolerance labeled a part""s response tolerance. An allowable tolerance constitutes an acceptable deviation from the target temperature profile for the part. The trial and error method can be time consuming and requires an experienced operator to implement.
In an improvement to the earlier procedure, guessing the control adjustment may be replaced with a computer executed algorithm that computes a part temperature difference profile between the target and measured values, and uses this difference profile in another algorithm that provides a series of changes to the control parameters in order to bring the measured part temperature profile closer to its target temperature profile. By successive iteration, control parameters to achieve allowable part response tolerances may be established within two to five attemptsxe2x80x94more quickly than through trial and error. The algorithms used in that method, however, require knowledge of the material properties of the board along with information on the nature of air current It within the zone, limiting the method""s practicality and complicating its implementation.
In an improvement to the earlier procedure, a feedback mechanism may be included which compares measurement-based data against a series of specified tolerances to indicate whether the part temperature responses satisfy the required conditions imposed by the part manufacturer. The numerical sign for whether the required conditions are satisfied may be called the Process Window Index, which represents a nondimensional measure of a part temperature profile to identify whether that profile satisfies the specification imposed by manufacturing requirements. It provides the operator a quantitative indicator of whether the control parameters require adjustment for production-mode thermal processing.
The Process Window Index, S, is a nondimensional positive real number described by a series of measured or measurement-derived parameters Bi numbering from i=1 to m each compared to its allowable tolerance band that the value for Bi must be within to satisfy the required conditions. These measured or measurement-derived parameters, which are hereafter called calculated feedback parameters, form a plurality of data values. One example of a measured parameter for Bi is a peak temperature of the part Tppeak as it moves along the conveyor in the reflow oven. A peak temperature on a part may be measured on at least one location on the part, and typically a plurality of temperatures may be obtained at different locations on the part in order to monitor the spatial nonuniformity of the part""s transient response.
An example of a measurement-derived parameter for Bi would be a maximum of part temperature change with respect to time which may be called part temperature change rate labeled as ∂Tp/∂t. (The xe2x80x9c∂xe2x80x9d sign represents the partial derivative in differential equations.) Such a quantity is typically not directly measured, but may be found in discretized form by subtracting part temperature measured at two separate times and dividing by the elapsed time between the measurements. The part temperature change rate may be typically monitored to minimize the risks of physical distortion of the part and maximize production throughput, and thus represents an important parameter to monitor. A second example of a measured-derived parameter used in electronics thermal processing applications is the xe2x80x9ctime above reflowxe2x80x9d tar: (Tpxe2x89xa7Tliq) meaning the period during which the part temperature has reached or exceeded the solder liquefaction temperature. This time above reflow may be monitored so as to better enable adequate liquification of the solder for a proper electrical connection between the printed circuit board and its mounted components, without damaging that board from excessive exposure to elevated temperatures.
An allowable part response tolerance band may be described by its maximum and minimum values. The Process Window Index represents the largest excursion relative to the tolerance bands from the series of calculated feedback parameters. An expression for the Process Window Index S may thus be shown as equation (2):
S=max{|(Bixe2x88x92xcex2+i)|/xcex2xe2x88x92i∀i=1, . . . , m}xe2x80x83xe2x80x83(2)
where xcex2+i=(Bimax+Bimin)/2 or midpoint within the tolerance band and xcex2xe2x88x92i=(Bimaxxe2x88x92Bimin)/2 or allowed excursion from the midpoint. (The symbol ∀ means xe2x80x9cfor allxe2x80x9d.) The values Bimax and Bimin represent the maximum and minimum limits within the tolerance band from the target value (typically the midpoint xcex2+i) for the feedback parameters Bi. The Process Window Index is the maximum normalized absolute value of the deviation with respect to the tolerance bands of all feedback parameters, and when displayed may be multiplied by one-hundred to provide a percentage value for the amount of tolerance xe2x80x9cwindowxe2x80x9d that is taken in the test process. This represents the typical form by which Process Window Index is provided to the operator. In order to satisfy the specification requirements, the percentage yielding Process Window Index in this percentage form should have an absolute value of less than one-hundred.
The centered capability ratio Cpk represents a complimentary parameter to the Process Window Index. Unlike the Process Window Index that determines thermal processor compliance to the part temperature profile from the specification requirements, the centered capability ratio comprises the lesser of the lower and upper data deviation boundaries. For example, let Bimin represent the lower bound of a feedback parameter tolerance band, Bimax the upper bound, {overscore (B)}i the average of the data for the ith feedback parameter and "sgr"B is the standard deviation. Then the centered capability ratio may be expressed as equation (3):
Cpk=min{(Bimaxxe2x88x92{overscore (B)}i)/3"sgr"B, ({overscore (B)}ixe2x88x92Bimin)/3"sgr"B}xe2x80x83xe2x80x83(3)
Typically, a Cpk value of 1.33 or higher may be desired, as indicating the data are not changing unexpectedly.
In the procedure to obtain the Process Window Index, calculated feedback parameters are compared in the above-described normalized form, and include parameters such as peak rise rate in the part temperature profile, time period that the part temperature profile is within a specified temperature range, and part temperature profile peak value within the reflow oven. Individual parameters may represent the dominant characteristic in local portions of the reflow oven. For example, part temperature rise rate generally reaches its peak in the initial zones, since the temperature difference between the part initially at room temperature and the first reflow oven zones that the part encounters may often be higher than the temperature difference between the partially heated part and the subsequent elevated control temperature zones. The peak value of part temperature may be typically attained towards the end of the reflow process where the control temperature setting may be highest.
Examination of the Process Window Index enables an operator to determine whether adjustment of the control parameters for the thermal process may be required. However, the solutions to improve the Process Window Index towards a lower value need not be unique, and so the operator must choose from several options, often based on arbitrary criteria. The additional requirement of having the operator input the control adjustment also affords an opportunity for control errors to be introduced.
One method by which part temperature response might be characterized absent measured test data beyond that acquired during the test process may be by prediction. FIG. 3 illustrates differences between discretized temperature values for the measured part and processor temperature profiles from the test process, which are plotted on a graph with temperature as the ordinate 50 and distance along the oven as the abscissa 52. In this example, the zones 54a and 54b extend across the oven distance. The control temperatures would correspond to the first two control parameters. The control temperature profile 56 showing the commanded values, processor temperature profile 58 reflecting the oven response, and the part temperature profile 60 featuring the part""s thermal response to oven exposure are plotted on the graph together. These profiles may be sampled for specific points along the distance axis. A part temperature Tp(t) point 80 on the part temperature profile 60 may be selected at interval "igr" for a particular distance value 82, with the temperature value corresponding to this point 80 associated with a level 84. At the next interval "igr"+1, another part temperature Tp("igr"+1) point 86 may be selected at distance 88 and temperature level 90. The distance increment 92 lies between intervals "igr" and "igr"+1. The change in part temperature between levels 84 and 90 may be identified as a part response difference 94 expressed as Tp("igr"+1)xe2x88x92Tp("igr") across that distance increment 92.
A processor temperature Tz("igr") point 96 on processor temperature profile 58 may also be selected at interval "igr", corresponding to distance 88 and temperature level 98. The control temperature Tc("igr") may also be identified at this distance 88 at interval "igr". The difference between the processor temperature level 98 and the part temperature level 84 for interval "igr" at distance 88 may be denoted in the form Tz("igr")xe2x88x92Tp("igr") as a thermal driver difference 100. The response difference 94 may be divided by the driver difference 100 to produce a temperature difference ratio labeled Rt and may be expressed at interval "igr" in equation (4):
Rt("igr")={Tp("igr"+1)xe2x88x92Tp("igr")}/{Tz("igr")xe2x88x92Tp("igr")}xe2x80x83xe2x80x83(4)
Alternate formulas may be used to correlate between part temperature transient response and the temperature difference between processor and part. Those of ordinary skill in the art will readily recognize that a plurality of correlation options are available.
Part response temperature also depends on conveyor speed as the third control parameter for the example discussed above. The conveyor speed governs exposure time of the part within each zone. If the conveyor speed is altered between the test process and the predicted process, the temperature difference ratio Rt may be multiplied by the conveyor speed ratio defined by Rs=[uc/ucxe2x80x2]b where uc is the conveyor speed during the measured thermal process, ucxe2x80x2 is the new speed for the response temperature prediction, and b is the power exponent. Typically this exponent b may be unity for a wide variety of electronic part designs. If the conveyor speed ratio Rs is constant throughout a thermal process and it may be treated as a scalar, unlike the temperature difference ratio Rt("igr") that may be separately calculated for each increment "igr". Otherwise it may be expressed as a ratio series.
Another controllable factor for part response temperature may be the fan speed, which governs the rate of convective heat transfer. While a convective heat transfer coefficient may be derived using analytical tools well known in the art, such techniques typically require measured data which are subject to large uncertainties and/or more difficult to obtain in a thermal processor than temperature values. Consequently, a cubic spline function may be used to approximate the functional relationship between the rate of change in part temperature and the conveyor speed, which may be either controlled and/or measured. For example, the part temperature rate change as it varies along the thermal processor may be expressed for a one-dimensional relation in equation (5):
∂Tp/∂t=ƒ(uc)≅xcex71uc3+xcex72uc2+xcex73uc+xcex74xe2x80x83xe2x80x83(5)
where xcfx89 represents the angular fan velocity and ƒ expresses a cubic spline relation defined such that first and second derivatives ƒxe2x80x2(uc) and ƒxe2x80x3(uc)are continuous, and the xcex7 coefficients in the cubic expression are solved by methods well known in the art.
Another measure to characterize the thermal processor uses a comparison between the zone temperature and the control temperature. This may be expressed as a processor ratio Rz of zone temperature to control temperature (both in absolute value, such as degrees Rankine or Kelvin) such as Rz("igr")=Tz("igr")/Tc("igr"), and may be calculated for each increment "igr". While the processor ratio at a particular increment may be stable for one set of conditions, a change from these conditions may alter the processor ratio, even after thermal equilibrium has been achieved, particularly if the change dramatically increases the difference between adjacent increments, such as across zones.
After completing the temperature difference ratio series for the test process in a conveyorized thermal processor at conveyor speed uc, the responding part temperature profile may be calculated for different control temperature profiles by a prediction process. The boundary conditions for the predicted process may be a new series of processor temperatures Tzxe2x80x2 and/or a new conveyor speed ucxe2x80x2. A starting value of the part temperature defines the initial condition (generally at ambient conditions) as Tpxe2x80x2(0), that is at "igr" equals zero. Then the predicted part temperature response at each subsequent interval may be determined from the relation in equation (6):
Tpxe2x80x2("igr"+1)=Tpxe2x80x2("igr")+{Tzxe2x80x2("igr")xe2x88x92Tpxe2x80x2("igr")}Rt("igr")Rsxe2x80x83xe2x80x83(6)
If the new conveyor speed is identical to the test conveyor speed, the speed ratio is unity and can be ignored in the process part temperature relation. By starting this explicit marching calculation at "igr" equals zero, the entire response temperature series for the given control profile can be determined in what may be termed an AutoPredict(trademark) process.
One major deficiency with the current thermal processor control is the absence of compensation for noncontrollable influence parameters, which may shift, for example, the processor ratio Rz so that even when the control parameters are held fixed, the part is exposed to a thermal environment that may not be constant over time. Such complexities increase the control uncertainty that a particular control series, selected to provide a thermal environment that enables the part temperature response to conform to the specification, will indeed satisfy those conditions.
The Process Window Index, AutoPredict process and other past developments provide instruments to indicate specification compliance or anticipate whether such It condition may be expected with which to guide an operator. However, no direct method or mechanism is currently available to directly correlate this feedback and/or prediction information with which to directly adjust control parameter settings during or after one or more parts pass through a thermal processor. Such a development would greatly expedite the production of parts in a thermal processor by eliminating repeated testing of parts to adjust the control parameters once the thermal processor is characterized.
A method and apparatus for controlling the temperature response profile of a part being exposed to heating and/or cooling conditions in a thermal processor incorporates measured data from the part to adjust the thermal processor control settings by closed loop feedback.